The present invention relates to cleaning processes that are used to remove residue from soldering components to printed wiring boards. More particularly, the present invention relates to an apparatus and method for evaluating the effectiveness of a selected cleaning process for cleaning printed circuit boards after the soldering process is complete. The manufacture of circuit card assemblies includes the step of adding separately manufactured component parts to the printed circuit board or printed wiring board (PWB). These component parts such as leadless ceramic chip carriers are often attached to the printed circuit board with an adhesive and then soldered to the printed circuit board. The component parts together with the printed circuit board form a circuit card assembly that is usually cleaned after the soldering process to remove excess solder and solder flux used to treat both the components and the printed wiring board for soldering. Failure to properly clean the printed circuit card assembly can result in solder shorts between the component leads and corrosion, both of which can cause long-term reliability problems.
Corrosion may occur if the circuit card assembly is not thoroughly cleaned. The flux used in the soldering process normally contain organic acids and activators containing halides. If the cleaning process is not adequate, traces of the chlorine, fluorine or bromine will be left along with the cleaning fluid. The introduction of moisture to an improperly-cleaned circuit card assembly can result in corrosion. Moisture may be introduced to the circuit card assembly in a variety of ways including humidity of the surrounding environment or condensation resulting from extreme temperature change.
Manufacturers of military electronics are often required to use specified fluxes during soldering operations such as rosin-based R or RMA fluxes. The removal of rosin-based R or RMA fluxes requires that the cleaning system make use of a solvent based on CFC-113 and methyl-chloroform. However, for environmental reasons manufacturers of military electronics have been either encouraged or required to make use of alternate solvents that do not contain chlorofluoro-hydrocarbons (CFC). Therefore, experimentation with cleaning solvents that do not contain CFC's is necessary so that alternate cleaning solutions may be identified and evaluated.
In addition to environmental concerns, experimentation with cleaning processes and cleaning solvents are often conducted to maximize the reliability of the completed circuit card assembly. Experimentation with the cleaning process is particularly important if changes are made to the circuit card assembly. These changes may involve the incorporation of newer technologies such as new electrical components that tend to have larger packages having larger numbers of leads and also fine pitch leads. Other changes to the circuit card assembly that may require adjustments to the cleaning process include new adhesives for attaching components and new solder fluxes for component cleaning. Therefore, to maximize the reliability of the completed circuit card assembly, experimentation is necessary with various cleaning processes and cleaning solutions so that excess solder or solder balls are eliminated, and corrosion is eliminated or minimized.
A known method for cleaning process control is described in U.S. Pat. No. 4,905,371 to Pai and assigned to the assignee of the present invention and incorporated herein by reference. Pai discloses the attachment of electrical components to a glass printed wiring board using a conventional soldering process. After attaching the components to the printed wire circuit board, the printed wire circuit board is cleaned using a selected process. Once the assembly and cleaning are completed, the effectiveness of the cleaning process is determined by visual inspection by peering through the glass printed wire board. The component is then desoldered from the glass printed wiring board and the adhesive is removed with a solvent. The components may be reused approximately ten times.
There is an ever present need for improved methods and apparatus by which the cleaning processes of the circuit card assembly can be tested in a cost-effective manner to determine the effectiveness of the cleaning process and to evaluate various cleaning solutions.